Looking inside our galaxy’s biggest black hole

Next month, astronomers will harness radio telescopes across the globe to create the equivalent of a single Earth-spanning dish—an instrument powerful enough, they hope, to image black holes backlit by the incandescent gas swirling around them. Their targets are the supermassive black hole at the heart of our Milky Way galaxy, known as Sagittarius A* (Sgr A*), and an even bigger one in the neighboring galaxy M87.

Earlier observations using this Event Horizon Telescope (EHT) without its full roster of dishes yielded tantalizing results, but in images the two black holes remained featureless blobs. This year, for the first time, the EHT will add dishes in Chile and Antarctica, sharpening its resolution and raising expectations. Astronomers now hope to see how the black holes whip the hot gas around them into accretion disks and spawn matter-spewing jets. They also hope to chart the size and shape of the event horizon—the boundary of the black hole—to test whether Albert Einstein’s theory of gravity, general relativity, still works under such extreme conditions.

“It’s a very bold and gutsy experiment,” says theoretical astrophysicist Roger Blandford of Stanford University in Palo Alto, California, who is not involved in the project. Blandford believes the EHT may not only show how black holes work, but also deliver a more fundamental message. “It will validate this remarkable proposition: that black holes are common in the universe. Seeing is believing.”

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At optical wavelengths, Sgr A* is hidden by the shroud of dust and gas obscuring the galaxy’s heart. Radio waves can pass through more easily, but ordinary radio dishes are still hampered by ionized gas clouds and low resolution. Best are telescopes sensitive to the shortest radio waves—millimeter waves—but the dishes, detectors, and data processing technology for this part of the spectrum were developed only in the past few decades. “There is only a tiny window where we can see the event horizon,” says Heino Falcke, an astrophysicist at Radboud University in Nijmegen, the Netherlands, and chair of the EHT science council. “The Milky Way is like a milky glass.”

Something to Believe In

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"Is it a fact—or have I dreamt it—that, by means of electricity, the world of matter has become a great nerve, vibrating thousands of miles in a breathless point of time?"— Nathaniel Hawthorne, The House of the Seven Gables, 1851